The method for producing multiwalled carbon nanotubes include chemical vapor deposition methods in which hydrocarbon and the like is thermally decomposed on catalyst metal to form multiwalled carbon nanotubes, and physical vapor deposition methods in which graphite is allowed to undergo sublimation by arc or laser to form multiwalled carbon nanotubes by the cooling process. The chemical vapor deposition methods are a method suited for large-scale synthesis since it is comparatively easy to scale-up a reactor.
The chemical vapor deposition methods can be roughly classified into two methods. One is a method in which a solution, prepared by dissolving metal compounds serving as a catalyst or a co-catalyst such as sulfur in a hydrocarbon such as benzene, is supplied to reaction field heated at 1,000° C. or higher using hydrogen as carrier gas, and formation of catalysts and growth of multiwalled carbon nanotubes are performed in the field (floating catalyst method). The other one is a method in which a supported catalyst in which catalyst metals or precursors are supported on carrier prepared in advance is placed in the reaction field heated at 500 to 700° C., and mixed gas of hydrocarbon such as ethylene with hydrogen or nitrogen is supplied and then reacted (supported catalyst method).
Since the reaction is performed in high temperature range of 1,000° C. or higher in the floating catalyst method, not only decomposition of the hydrocarbon on the catalyst metals but also an autolysis reaction of hydrocarbon proceeds. Pyrolytic carbon is deposited on the multiwalled carbon nanotube grown from the catalyst metal as the starting point, and the nanotube also grows in the thickness direction of the fiber. The multiwalled carbon nanotube obtained by this method (hereinafter, multiwalled carbon nanotubes having a fiber diameter of 50 nm or more synthesized by floating catalyst method is referred to as a carbon nanofiber) has comparatively low conductivity since it is coated with pyrolytic carbon having low crystallinity. Therefore, the multiwalled carbon nanotubes are synthesized by the floating catalyst method, and then graphitized by heat treatment in an inert gas atmosphere at a temperature of 2,600° C. or higher. The heat treatment enables proceeding of crystal rearrangement and graphite crystal growth, leading to an improvement in conductivity of the fiber. The heat treatment also enables vaporization of the catalyst metal to give carbon nanofibers with less impurity.
On the other hand, since the reaction is performed at 500 to 800° C. in the supported catalyst method, the autolysis reaction of the hydrocarbon is suppressed. It is possible to obtain thin multiwalled carbon nanotubes as a result of growing from the catalyst metals as the starting point. The obtained multiwalled carbon nanotubes have comparatively high crystallinity and comparatively high conductivity. Therefore, it is not necessary to perform the heat treatment for crystallization which is applied to the multiwalled carbon nanotubes obtained by the floating catalyst method. Since the multiwalled carbon nanotubes synthesized by the supported catalyst method is not subjected to the heat treatment at high temperature for crystallization, the catalyst metals remains in a percentage order in the multiwalled carbon nanotubes.